In a conventional engine control system, an injection-quantity command value (injector-opening-period command value), which indicates a fuel quantity injected by a fuel injector, is corrected by executing a small-injection-quantity learning which will be described below. That is, when the vehicle is decelerated without injecting fuel, a small quantity of fuel is compulsorily injected, whereby an engine speed NE is slightly increased. Based on an increase ΔNE in engine speed, an increase ΔTrq in engine output torque is computed. Further, based on the increase ΔTrq, an actual fuel injection quantity Qact can be computed. A deviation between the actual quantity Qact and the injector-opening-period command value is learned as an injection quantity correction value so that the injector-opening-period command value is corrected. This learning is referred to as a small-injection-quantity learning.
In order to execute the small-injection quantity learning, it is necessary to previously obtain a conversion factor for converting the increase ΔTrq into the injection quantity Qact by experiments. Further, since the conversion factor depends on an injection condition, such as a fuel supply pressure (pressure in a common-rail), an engine speed NE, a fuel temperature and the like, it is necessary to form a map of conversion factor with respect to every injection condition, which increases work load to form the map.
JP-2010-223182A, JP-2010-223183A, JP-2010-223184A and JP-2010-223185A respectively show a fuel injection system which is provided with a fuel pressure sensor detecting a fuel pressure in a fuel passage between a common-rail and an injection port of a fuel injector. Based on a detection value of the fuel pressure sensor, a fuel pressure waveform indicative of a variation in fuel pressure due to a fuel injection is detected. According to this system, since the injection-rate waveform indicative of the injection-rate can be computed based on the detected fuel pressure waveform, the injection quantity can be computed based on an area of the injection-rate waveform. That is, since the actual injection quantity is directly detected by a fuel pressure sensor, it is unnecessary to execute the correction based on the small-injection quantity learning, whereby it is unnecessary to form the map of conversion factor.
However, in a case that the above system is applied to a multi-cylinder engine, it is necessary that the fuel pressure sensor is provided to each of fuel injectors, which may increase its costs.
If only specified fuel injectors have the fuel pressure sensor, the number of the fuel injectors can be reduced. However, it becomes necessary to execute the above small-injection quantity learning with respect to the fuel injectors having no fuel pressure sensor, which increase a work load for forming the conversion factor map.